To be able to control or regulate the individual roll stands of a rolling train appropriately, reliable detection of the speed of the rolling stock is necessary.
To detect the speed of a metal strip the rotational movement of the working rollers of a roll stand or also of other rollers that are in contact with the rolling stock can be used for instance. However the problem which arises here is that the measurement signal must be corrected since slippage can occur in practice between working rollers or rollers and rolling stock respectively.
Non-contact measurement sensors for measuring the strip speed are known, such as for example from EP 1 252 942 A1. In such cases a laser beam is transmitted onto the train surface and the strip speed is determined from the Doppler shift of the reflected signal. Such an optical measurement system is however susceptible to faults. Since liquid coolant is needed for the cooling of working rollers, contact with the high-temperature of a hot strip results in the formation of steam. The propagation of the light beam is thus adversely affected and this results in inaccuracies in the speed measurement. Because of the harsh ambient conditions of a rolling train it can also occur that an optical measurement system fails completely.
A precise and reliable determination of the strip speed however also has other advantages, including the ability to determine the thickness of the metal strip in indirect ways from the strip speed, as is explained in more detail under FIG. 4.